Method for making ferrous metal having highly improved resistances to corrosion at elevated temperatures and to oxidization

ABSTRACT

A method of making ferrous metal having highly improved resistances to corrosion and to oxidization, which comprises a step of hot-dipping a metallic workpiece in a molten aluminum or aluminum alloy bath, a step of primary heat-treatment of the metallic workpiece and a step of secondary heat-treatment of the metallic workpiece. During the primary heat-treatment, an intermetallic compound layer is formed over the surface of the metallic workpiece, which is developed into an alloy layer during the secondary heat-treatment to make the alloy layer firmly secured to the base of the workpiece. The secondary heattreatment may be omitted depending on the application of the ferrous metal processed by this method.

United States Patent [191 Sasame et al.

[451 Sept. 23, 1975 [54] METHOD FOR MAKING FERROUS METAL HAVING HIGHLYIMPROVED RESISTANCES TO CORROSION AT ELEVATED TEMPERATURES AND TOOXIDIZATION [75] Inventors: Takao Sasame; Toshio Yagi, both ofHiroshima, Japan [73] Assignee: Toyo Kogyo Co., Ltd., Japan [22] Filed:Nov. 10, 1972 [21] Appl. No.: 305,558

[52] US. Cl. 148/6.3; 29/195; 29/1961; 29/1962; 148/31.5 [51] Int. Cl.C23C 11/00; B23P 3/00 [58] Field of Search 148/6.3, 6.35, 31.5; 117/114C;29/l96.1,196.2, 195

[56] References Cited UNITED STATES PATENTS 2,444,422 7/1948 Bradfordmlnl48/6.35 X

3,418,174 12/1968 Carter et al l48/6.35 3,660,173 5/1972 Matsumo et al.148/6.3 X 3,730,758 5/1973 Laidman 117/1 14 C X 3,779,056 12/1973 Radjenet al 117/114 C X Fe-Afilntermetolhc Compound Loyerfl -Bose 3,787,228l/1974 Rausch et al. 117/114 C FOREIGN PATENTS OR APPLICATIONS 754,5928/1956 United Kingdom 1,018,628 l/l966 United Kingdom 1,079,687 8/1967United Kingdom Primary ExaminerThomas J. Herbert, Jr. Attorney, Agent,or Firm-Wenderoth, Lind & Ponack [57] ABSTRACT A method of makingferrous metal having highly improved resistances to corrosion and tooxidization, which comprises a step of hot-dipping a metallic workpiecein a molten aluminum or aluminum alloy bath, a step of primaryheat-treatment of the metallic workpiece and a step of secondaryheat-treatment of the metallic workpiece. During the primaryheattreatment, an intermetallic compound layer is formed over thesurface of the metallic workpiece, which is developed into an alloylayer during the secondary heat-treatment to make the alloy layer firmlysecured to the base of the workpiece. The secondary heattre'atment maybe omitted depending on the application of the ferrous metal processedby this method.

6 Claims, 4 Drawing Figures F/GZ US Patent Sept. 23,1975 Sheet 10f23,907,611

Fe-Aelmermetollic Compound Layer-EL Bose *Bcse PAlloy Layer US PatentSept. 23,1975 Sheet 2 of2 3,907,611

FIG. 3

[A8 Fe-Mlntermetollic F Compound Loyer -\e Base k A -Cr CompoundContained w "at -A 203 (Fe, Cr)

CORROSION AT ELEVATED TEMPERATURES AND 'ITO OXIDIZATION The presentinvention relates to a method for makin'g ferrous metal'having highlyimproved'resistances' to corrosion at elevatedtemperaturesand tooxidization and, more particularly, to the method wherein'a ferrousmetal workpiece, after having been hot-dipped in a molten metal bathcontaining therein aluminum or its alloy with chromium, isheat-treatedto form an alloy layer 7 on the base of the ferrous metal workpiecethereby to improve theresistances to corrosion'at the elevatedtemperatures and to oxidization.-

In thefield of autornobile'industry, 'an' after-burner or thermalreactor has been largely employed in an automotive vehicle and ithas'been well recognized as one of effective means "for reducingorsubstantially eliminating noxious unburned compounds present inexhaustgas emerging from the exhaust system of the automotive'vehicle;As has been well understood, the exhaust gas emerging from theautomotiveexhaust system contains,'in addition to the unburned compounds such asCO and HC, gaseous halogens and halides and- /or lead compounds suchasClBr PbCl PbBr C l l Cl and C H Br which are corrosive mediums. In

the presence of such corrosive mediums, various metallic parts disposedin the automotive exhaust system tend to be easily corroded and, inthe'case where such an after-burnerin provided in theautomotive exhaustsystem, in which the interior temperature tends to elevate up to 900C.and, in an extreme ease, up to l,200C., during re-combustion of theunburned compounds-in saidafter-burner, corrosion of the afterburner isaccelerated by the elevated temperature, thus reducing the workingefficiency of the after-burner while shortening the service time of theafter-burner. In view of this, especially the after-burner is requiredmade of speciallydesigned, expensive material having a sufficientresistances to corrosiona't the elevated temperatures as well as tooxidization.

Recently, such a metallic material as having a relatively high corrosionresistance at elevated temperatures as wellas a sufficient resistance tooxidization has been developed in the field of space craft industry.which may be more or less satisfactorily employed in the manufacture ofthe after-burner or thermal reactor. However, this metallicmaterialdeveloped for the manufacture of a space craft is too expensive to makeit available for the manufacture of metallic articles for publicuseincluding an'automotive vehicle having such an after-burner. l

- Accordingly; an essential object of the present invention is toprovide an improved method for making ferrous metal having a relativelyhigh corrosion resistance and resistance to oxidization which can berelatively easily manufactured at low costs.

Another important object'of the present invention is to provide theimproved method of the above mentioned character wherein an article madeof ferrous metal is first hot-dipped'in a molten metal bath containingtherein aluminum or its alloy to form an aluminum foil over thesurface-of said article and, then, subjected to a heat-treatment to forman alloylayer on the base of the ferrous metal of said article-therebyto improve the corrosion and oxidization resistances of'said arti- LII 2cles without requiring a substantially complicated procedure. I

It is a related object of the present invention to providea ferrousmetal workpiece manufactured by the above mentioned method.

It is to be noted that the present invention has been primarilydeveloped in view ofproviding a relatively high corrosion andoxidization resistant and inexpensive metallic material for themanufacture of the afterburner. However, themethod of the presentinvention has many applications and, for example, it is applicable notonly to the manufacture of automotive vehicle parts such as after-burnerand exhaust muffler, but also to the manufacture of various metallicarticles such as plates, pipings, containers, vessels and others, all ofwhich are accessible to corrosive mediums no matter how said corrosivemediums are in gaseous, solid or liquid state and which are made offerrous metal.

Furthermore. in view of the fact that the ferrous metal processed by themethod of the present invention has a relatively high corrosionresistance as well as a relativelyhigh resistance to oxidization, themethod of the present invention can be applicable to the manufacture ofvarious articles which require either or both of these characteristics.Moreover, the corrosion resistance of the ferrous metal process by themethod of the present invention permits such ferrous metal to be used inthe manufacture of various jigs for use in casting of light alloys suchas those of aluminum and zinc, which may otherwise be easily oxidized incontact with the molten casting metal.

It is also to be noted that the present invention may be applicable toany type of metallic articles including unprocessed workpieces producedby such methods as rolling, extrusion, drawing, casting and others andintermediate and final products processed by such methods as pressing,welding, grinding and others. In addition thereto, the unprocessed,intermediate or final products, after having been processed by themethod of the present invention, may be clectro-plated in any suitablemanner.

These and other objects and features of the present invention willbecome apparent from the following description taken in conjunction witha preferred embodiment thereof with reference to the accompanyingdrawings, in which;

FIG. 1 is a schematic diagram showing the longitudi nal section of aferrous workpiece obtained by dipping in a molten metal bath containingsubstantially pure aluminum and then heat-treating according to thepresent invention and a graph obtained by the X-ray 'microanalizer ofdistribution of various elements contained in the ferrous workpiece thusprocessed,

FIG. 2 is a similar diagram to FIG. 1, but the ferrous workpiece beingfurther heat-treated,

FIG. 3 is a schematic diagram showing the longitudinal section ofanother ferrous workpiece obtained by dipping in a molten metal bathcontaining an aluminum alloy including 4 wt% of chromium and thenheattreating according to the present invention and a graph obtained bythe X-ray microanalizer of distribution of various elements contained inthe ferrous workpiece thus processed, and

FIG. 4 is a similar diagram of FIG. 3, but the ferrous workpiece beingfurther heat-treated.

Before the description of the present invention proceeds,it is to benoted that, although various elements contained in an article made offerrous metal (hereinafter referred to as ferrous workpiece), forexample, Cr and Ni in the case where such ferrous workpiece is made inaustenitic stainless steel, are in practice distributed in a layerformed on the surface of said workpiece during execution of the methodofthe present invention, more or less affecting the characteristics ofsaid layer, the distribution of such elements is not shown in theaccompanying drawings because the present invention is to be understoodin terms of a relationship between a ferrous substance contained in theworkpiece and an aluminum substance and/or a chromium substancecontained in the layer.

It is further to be noted that the method of the present invention canbe applicable to any workpiece made of any one of ferrous steel,including various carbon steels and such special steels as ferriticstainless steel, martensitic stainless steel and austenitic stainlesssteel, and cast ironsincluding ordinary cast irons and such special castirons as ductile cast iron and alloy cast iron.

In other words, it is to be understood that any workpiece made ofmaterial having Fe as the principle component can be subjected to themethod of the present invention no matter what it contains, in additionto Fe, one or a mixture of numerous inorganic additives.

As the first step of execution of the method of the present invention,the ferrous workpiece is hot-dipped for 30 to 300 seconds in a moltenmetal bath containing therein aluminum or its alloy and heated to atemperature of from 700 to 950C, thereby to form a metallic foil on thesurface thereof. The reason for the employment of aluminum or its alloyfor the moltenmetal bath is because of its excellent corrosion andoxidization resistances. f

In this case, if the bath temperature is lower than the lowermost limitof 700C., a satisfactory result of dipping process cannot be obtainedand, if it is, higher than the uppermost limit of 950C, dimensionalvariation of the ferrous workpiece will be observable due to softeningof the ferrous workpiece under the elevated temperature and, in anextreme case, it will be partially or wholly melted into the moltenmetal bath.

Preferably, the use of aluminum alloy is recommended for the moltenmetal bath, in which case the aluminum alloy must contain chromium in anamount within the range of from 1 to percent by weight based on thetotal weight of the aluminum alloy. This is because the presence ofchromium in the molten metal bath results in an improvement in thesmoothness of the surface of the resultant foil adhering to the ferrousworkpiece and also in the corrosion resistance with respect to any oflead compounds. However, if the aluminum alloy including chromium in anamount smaller than the lowermost limit of l wt7r is employed, suchimprovements as hereinabove described cannot be obtained whereas, if thealloy including chromium in an amount greater than the uppermost limitof 10 wt% is employed, the surface of a compound layer as will bementioned later will be roughened at the time of completion of thesubsequent heat-treatment. It is to be noted that, in the case where themolten metal bath is prepared by the use of the aluminum alloy includingchromium in the specified amount, the lowermost limit of the bathtemperature be preferably fixed at 750C.

With the above in mind, if the dipping hour is shorter than thelowermost limit of 30 seconds, the subsequent heat-treatment subject tothe ferrous workpiece having the metallic foil thereon will not result,in satisfactory formation of the compound layer that is, an Al-Feintermetallic .compoundlayer containing a Fe-Al compound, such asFe Aland Fe Al, as the principle component in a desired depth, whereas, if itis longer than the uppermost limit of 300 seconds, a ferrous substancecontained in the ferrous workpiece will be melted into the molten bath,thus reducing the weight of the workpiece to be processed. The dippinghour may vary, within the range of 30 to 300 seconds, depending uponthethickness and shape of the ferrous workpiece to be processed by themethod of the present invention.

Subsequent to the hot-dipping process, the ferrous workpiece having ametallic foil thereon is thensubjected to a primary heat-treatmentthereby to cause the aluminum substance in the metallic foil to undergoan intermetallic combination with the ferrous substance in the ferrousworkpiece. More specifically, if the substantially pure aluminum isemployed for the molten metal bath, only the Fe-Al intermetalliccompound layer of 50 to 200 micron in depth which contains the Fe-Alcompound as the principle component can be obtained as shown in FIG. 1.On the other hand, if the aluminum alloy is employed for the moltenmetal bath, as shown in FIG. 3, not onlythe Fe-Al intermetallic compoundlayer of 50 to 200 micron in depth can be obtained, but also the sameintermetallic compound layer contains, in addition to the Fe-Al compoundas the principle component, an Al-Cr compound, such as Al Cr and Al-,Cr, distributed substantially in the vicinity of the surface of saidintermetallic compound layer with the aluminum substance being stablizedtherein.

To achieve theoptimum result, the primary heattreatment must be carriedout under a temperature of from 700 to 930C. for more than 30 minutes.If the heating temperature is lower than the lowermost limit of 700C,the intermetallic compound layer will not be satisfactorily obtainedand, if it is higher than the uppermost limit of,930C., the aluminumsubstance contained in the foil will not satisfactorily undergo theintermetallic combination with the ferrous substance in the ferrousworkpiece during this primary heat treatment and, hence, a satisfactorydevelopment of the intermetallic compound layer will not be observable,because of being oxidized into A1 0 without providing the desiredcorrosion resistance. The heating temperature-may vary, within the rangeof from 700 to 930C, depending upon the type or kind of the ferrousworkpiece to be processed by the method of the present invention. I

In addition, the heating time must be not less than 30 minutes, orotherwise development of the intermetallic compound layer will not besatisfactorily observed. However, the heating may preferably continuefor not more than 3 hours in view of economical industrial practice.

This primary heat-treatment may be carried out in any ambient atmosphereexcept for extremely oxidizing atmosphere.

The art of hot-dipping a ferrous workpiece in the molten metal bathcontaining aluminum or its alloy for the purpose of improving theresistance to oxidization is heretofore known by those skilled in theart. However, the faet has not yet been known that the resistance tocorrosion at the elevated temperature of the ferrous workpiece afterhaving been dipped in the molten metal bath can be improved byheat-treatment under the temperature of from 700 to 930C. for more than30 minutes. In other words. because a ferrous articlemanufactured merelyby dipping it in the molten metal bath is such that the aluminumsubstance contained in the resultant foil is not distributed so as tocombine with the ferrous substancesthe article lacks the sufficientcorrosion resistance. Fromthe foregoing. it is clear that the mostimportant feature of the present invention so far described resides inthat the ferrous workpiece is. after having been hot-dipped in themolten metal bath. subjected to the heat-treatment under the particulartemperature for the particular period of time.

It is also clear that the aluminum substance transplanted to the surfaceof the ferrous workpiece during the hot-dipping process is uniformlydistributed into the workpiece during the primary heat-treatment toprovide the intermetallic compound layer which has relatively highcorrosion and oxidization resistances. However, this intermetalliccompound layer has a somewhat insufficient interlocking with respect tothe base of the ferrous workpiece.

Accordingly. in the case where the ferrous workpiece having theintermetallic compound layer is to be used in contact with flowingparticles of gaseous corrosive medium and/or to be used in themanufacture of an ar ticle accessible to vibrations. it is recommendedto carry out a secondary heat-treatment to permit the intermetalliccompound layer to be firmly interlocked with the base of the workpiece.i.e.. to develop into an alloy layer. without substantially reducing theimproved corrosion and oxidization resistances.

This secondary heat'treatment is carried out at a temperature lower thanthe melting point of the ferrous workpiece and within the range of 950to l.350C. for not more than hours. The primary purpose of thissecondary heat-treatment is to diffuse aluminum. which is one of thesubstances contained in the intermetallic compound layer. into theferrous workpiece thereby to form the alloy layer having Fe and Al asits principle composition which is firmly interlocked with the base ofthe ferrous workpiece. During this process. there can be found atendency that. as diffusion of aluminum proceeds. the content ofaluminum in the surface portion of the resultant alloy layer reduceswith the result of reduction of the corrosion resistance. In order toavoid this tendency. the secondary heattreatment must be carried out inthe oxidizing atmosphere.

If the secondary heat-treatment is carried out in the oxidizingatmosphere as hereinbefore described. the aluminum present in thesurface portion of the resultant alloy layer is combined with oxidepresent in the acidic atmosphere thereby to form a compound such as aAl-,O without reducing the density of aluminum present in said surfaceportion. In addition thereto. in view of the fact that the primaryheat-treatment is cf fective to form the Fe-Al intermctallic compoundlayer. Fe acts as a binder. even if the aluminum present in said surfaceportion is oxidized as hereinbefore described. so that a layer ofCK-AI2O3 compound can be firmly secured in the resultant alloy layer.Accordingly. the improved corrosion and oxidization resistancesobtainable by the method of the present invention does not substantiallyreduce. It is to be noted that. in the case where the molten metal bathcontain the aluminum alloy including chromium. chromium also acts as abinder in the surface portion of the resultant alloy layer incooperation with Fe.

To achieve the optimum result ofthe secondary heattreatment. the heatingtemperature must be within the range of from 950 to l.350C. as statedabove. If this temperature is lower than the lowermost limit of 950C.satisfactory diffusion of the aluminum will not be observable and.hence. the alloy layer containing Fe and Al as its principle componentswill not be satisfactorily formedwith no substantial improvement of theinterlocking with the base ofthe ferrous workpiece. On the other hand.ifthe temperature is higher than the up pcrmost limit of 1.350C.. notonly the aluminum present in the surface portion of the resultant alloylayer is oxidized into a-AI- O but also Fe. which acts as a binder. isoxidized and, therefore. the corrosion resistance and the resistance tooxidization will be reduced. In any event. this heating temperature mayvary. within the range of from 950 to l.350C.. depending upon themelting point of the ferrous workpiece to be processed. In other words.the heating temperature must be lower than the melting point of theworkpiece to be processed. For example. in the case where the ferrousworkpiece is made of cast iron. it must be lower than l.l00C.

In addition thereto. the heating hour must be not more than 10 hours. orotherwise Al and Fe are similarly oxidized. respectively. withsubstantial reduction .of the corrosion resistance as well as theresistance to oxidization. The minimum permissible heating hour duringthe secondary heat-treatment is not limited because of the fact that theferrous workpiece that has been subjected up to the step of primaryheattreatment can be placed in practical use without subjecting to thesecondary heat-treatment. However. in the case where the secondaryheat-treatment in the manner as hereinbcfore described is to be carriedout. at least 30 minutes or more is required. It seems. from theforegoing. that the heating hour is limited within the range of 30minutes to 10 hours and this is' true in the case where the secondaryheat-treatment is carried out in the usual atmosphere. However. in thecase where the secondary heat-treatment is carried out in the oxidizingatmosphere. the heating hour may be shorter than the minimum permissiblevalue of 30 minutcs.

The alloy layer thus obtained on the ferrous workpiece upon completionof the secondary heattreatment represents such structures as shown inFIGS. 2 and 4 and each of which extends toward the base in a depth offrom 200 to 500 micron. the structure of FIG. 2 being the case where theferrous workpiece has been dipped in the molten metal bath ofsubstantially pure aluminum while the structure of FIG. 4 being the casewhere the ferrous workpiece has been dipped in the molten metal bath ofthe aluminum alloy with chromium.

From FIG. 2, it is clear that the alloy layer contains (it-A1 0 rigidlysecured by the binding action of Fe in the vicinity of the surfaceportion thereof with the aluminum content reducing toward the base. Thestructure of FIG. 4 is similar to that of FIG. 2, but the ot-Al Ocompound is rigidly secured by the binding ac tion of Fe and Cr whilethe contents of Al and Cr respcctively reduce toward the base. Moreparticularly. in the structure of FIG. 4, the alloy layer comprises asurface portion containing Fe. Al and Cr and a portion adjacent to thebase containing Fe and Al.

In either of the structures of FIGS. 2 and 4. the depth of the surfaceportion of the alloy layer including the (X-Al is approximately to 80micron.

From the foregoing. it has now become clear that. by the secondaryheat-treatment, (It-Alg0 is formed in the surface portion of the alloylayer as secured by the binding action of Fe and. consequently. theimproved corrosion and oxidization resistances can be obtained. In thecase where the alloy layer contain chromium because of the use of thealuminum alloy for the molten metal bath, not only this chromiumcooperate with Fe as a binder, but also the presence of chromiumimproves the corrosion resistance to the lead compounds and,accordingly, this ferrous material having the alloy layer containingchromium can be advantageously utilized in the manufacture of theafter-burner and exhaust muffler. Furthermore, the secondaryheattreatment permits aluminum to diffuse into the ferrous taining, inaddition to Fe and Al as its principle components, such elements as usedas the additives. Accordingly, no substantial reduction of the corrosionand oxidization resistances occur even in the presence of the additivesin the ferrous workpiece. Byway of example, in the case where theferrous workpiece is made as austentic stainless steel, chromium andnickel contained in According to the present invention, it has beenfound that, if the secondary heat-treatment is carried out subsequent tothe primary heat-treatment which has been carried out under thetemperature within the range of 700 to 930C. for more than minutesthereby to permit the aluminum to be stabilized, formation of a-AI Q,takes place in an individually scattered manner and, consequently, nosubstantial peeling of the alloy layer as hereinbefore described takesplace.

Hereinafter, the present invention will be illustrated by way ofexample.

said austentic stainless steel are diffused into the alloy layer duringthe secondary heat-treatment, thereby forming the alloy layercontainingsuch elements in adstantially intermediate portion of theresultant alloy layer during the secondary heat-treatment. If this for-Five groups of four sample pieces A, B, C and D were tested. These foursample pieces, A, B, C and D are made of ductile cast iron (globulargraphite cast iron), low carbon steel containing 0.1% of carbon, I7Crsteel (one of ferritie stainless steels) and l8Cr-8Ni steel (one ofaustenitic stainless steels). It is to be noted that l7Cr steel isspecified by "SUS 24" according to the Japanese Industrial Standard G4304-9 which is an equivalent of l7Cr steel as specified by Type No.430" according to the American Iron and Steel Institute or X8 Crl7f asspecified by No. 4016 according to Deutsche Industrie Normung, andl8Cr-8Ni steel is specified by SUS 27" according to the JapaneseIndustrial Standard G 4303-9 which is an equivalent of 18-88" asspecified by Type No. 304" according to the American Iron and SteelInstitute or X5 CrNi l8 9 as specified by No. 430] according to DeutscheIndustrie Normung.

The test 'resuts are shown in the following Table wherein the fivegroups are clasified by I. II, III, IV and V. Of them, the four. samplepieces A, B. C and D under Group I were submitted to the test withoutbeing processed by the method of present invention while those underGroups II and V were processed by the method of-the present invention asexemplified in the Table mation of a-Al o takes place in a relativelygreat prior to the test.

TABLE Primary Heat- Secondary Heat- Weight Reduction Rate Sample DippingProcess Treatment under Treatment under (71) Piece Bath Temp. Type ofAtmosphere Atmosphere at at at Groups Codes (C) Molten Metal Temp.(C.)Hour Temp.(C.) Hour 900C. l.000C. l.l00C.

A 32 60 B 25 I Not Processed Not Treated Not Treated C I5 24 78 D l3 2I58 A 800 I Not Treated 3 8 Pure B 750 aluminum 800 1 Not Treated 2 6 II1 minute dipped C 780 1.5 Not Treated l 4 [3 D 770 l 5 Not Treated 0 3 6A 800 l l .100 1.5 5 l3 Pure B 750 aluminum 800 I 1.100 2 3 ll Ill 1minute dipped C 780 L5 l,l()() 2 2 6 l7 D 770 1.5 H50 l l 5 7 Table-.Continued l'rimary Heat- Secondary Heat- Weight Reduction Rate SampleDipping Process Treatment under Treatment under- (71) Piece Bath Temp.Type of Atmosphere Atmosphere at at at .Groups Codes ("C) Molten'MetalTemp.(C.) Hour Tcmp.(C.) Hour 900C. 1.000C. 1.100C.

A 800 1 'Not Treated 3 aluminum alloy with 800 I Not Treated 0 2 IV- 8304 chromium C 1 minute dipped 780 1.5 Not Treated 0 0 l D 770 1.5 NotTreated 0 0 2 aluminum B alloy with 800 1 1.100 2 0 4 w V e 830 47:chromium C 1 minute clipped 780 1.5 1.100 2 0 0 2 D 770 1.5 1.150 1 O 02 The test were carried out in the following manner: Each sample piecesunder Groups 1 and V was prepared in the form of a plate having 3 mm inthickness.

30 mm in width and mm in depth and was horizontally placed on a testbench. A powder of lead halide was spread 2 mm in depth over the uppersurface of each of the sample pieces under Groups 1 to V and heated at apredetermined temperature of 900C. 1.000C..and 1.100C. for 2'hours. Eachof the test results is tabulated in the Table in termsof percentage ofreduction of the weight with respect to the original weight measuredprior to'the test.

As to the bondability'of the alloy layer in the'sample pieces A, B, Cand D under Groups 11 to V. these samunder-Groups 11 and 1V. Thisfactclearly indicates that the bondability of the alloy layer withrespect to the ferrous workpiece can be improved by performing thesecondary heat'treatment. irrespective of the presence of chromium inthe molten metal bath during the hotdipping process. I v

From the foregoing description, it has now become ,clear that thepresence of chromium in the aluminum bath. i.e.. the employment ofeither pure aluminum or the aluminum alloy for the molten metal bath,affects 1 the improvement in" the corrosion resistance and the secondaryheat-treatment in the method of the present invention improves thebondability of the alloy layer to "the ferrous workpiece compared withthat without the secondary heat-treatment. 1

In any case, the test results exhibit that the method workpiece ascompared with those not processed by the methodof the present invention.1 j J I s'pecificallyyferrous metallic meterial that has been dipped inthe molte'nmetal bath of the aluminum alloy. then subjected to theprimary heat-treatment and finally subjected to the secondaryheat-treatment is recommendable for the manufacture of the after-burnerwhich has the severe requirements as hercinbefore described.

resistance to oxidization whereas the presence of the 'ofthe presentinvention is-ve'ry effective to improve the corrosion and oxidizationresistances of the-ferrous.

What is claimed is:

1. A method for making ferrous metal having highly improved resistanceto corrosion and oxidization. which comprises dipping a ferrousworkpiece in a molten metal bath containing an aluminum alloy containingchromium in an amount within the range of l to 10 percent by weightbased on the total weight of said aluminum alloy. and having atemperature of from 750 to 930C for from to 300 seconds. heating theresultant ferrous workpiece for at least 30 minutes at an elevatedtemperature of from 700 to 930C. and heating the thus treated ferrousworkpiece at a predetermined termperature within the range of 950 to1.350C which is lower than the melting point of said ferrous workpiece.under an oxidizing atmosphere for not more than 10. hours. therebyforming an alloy layer uniformly over the surface of said ferrousworkpiece. said alloy later containing iron and aluminum as its principle components and including a substantial layer of a oz-Al O securedtherein by the binding action of said iron. at the surface portion ofsaid alloy laycr. said surface portion containing chromium which acts asa binder for securing said substantial layer of cit-A1 0 in cooperationwith said iron.

2. A method as claimed in claim 1, wherein said alloy layer is 200 to500 microns in depth with said substantial layer of oz-A1 0 extending 10to 80 mircrons in depth.

3. A method as claimed in claim 1. wherein the minimum heating timeduring the second heating step is more than 30 minutes.

4. A method for making ferrous metal having highly improved resistanceto corrosion and oxidization.

' which comprises coating a ferrous workpiece with an aluminum alloycontaining chromium in an amount within the range of l to 10 percent byweight based on the total weight of said aluminum alloy. heating thecoated ferrous workpiece for at least 30 minutes at an elevatedtemperature of from 700 to 930C. and heating the thus treated ferrousworkpiece at a predetermined temperature within the range of 950 tol.35(lC which is lower than the melting point of said ferrous workpiece.under an oxidizing atmosphere for not more than 10 hours. therebyforming an alloy layer uniformly over the surface of said ferrousworkpiece. said alloy layer containing iron and aluminum as itsprinciple components and including a substantial layer of a a-Al- Q,secured therein by the binding action of said iron. at the surfaceportion of said alloy layer. said surface portion containing chromiumwhich acts as a binder for securing said substantial layer (via-A1 0 incooperation with said iron.

5. A ferrous workpiece prepared according to the method of claim 1.having a surface portion formed with an alloy layer having relativelyhigh resistances to corrosion and to oxidization said alloy layer being200 to 500 microns in depth and containing iron and aluminum as itsprinciple components and including therein a substantial layer of a-Al Oof IO to 80'microns in depth secured in a surface portion of said alloylayer, and wherein at least said surface portion of said alloy layercontains chromium which acts as a binder for se-

1. A METHOD FOR MAKING FERROUS METAL HAVING HIGHLY IMPROVED RESISTANCETO CORROSION AND OXIDAZATION, WHICH COMPRISES DIPPING A FERROUSWORKPIECE IN A MOLTEN METAL BATH CONTAINING AN ALUMINUM ALLOY CONTAININCHROMIUN IN AN AMOUNT WITHIN THE RANGE OF 1 TO 10 PERCENT BY WEIGHTBASED ON THE TOTAL WEIGHT OF SAID ALUMINUM ALLOY, HAVING A TEMPERATUREOF FROM 750* TO 930*C FOR FROM 30 TO 300 SECONDS, HEATING THE RESULTANTFERROUS WORKPIECE FOR AT LEAST 30 MINUTES AT AN ELEVATED TEMPERATURE OFFROM 700* TO 930*C, AND HEATING THE THUS TREATED FERROUS WORKPIECE AT APREDETERMINED TEMPERATURE WITHIN THE RANGE OF 950* TO 1,350*C WHICH ISLOWER THAN THE MELTING POINT OF SAID FERROUS WORKPIECE, UNDER ANOXIDIZING ATMOSPHERE FOR NOT MORE THAN 10 HOURS, THEREBY FORMING ANALLOY LAYER UNIFORMLY OVER THE SURFACE OF SAID FERROUS WORKPIECE, SAIDALLOY LATER CONTAINING IRON AND ALUMINUM AS ITS PRINCIPLE COMPONENTS ANDINCLUDING A SUBSTANTIAL LAYER OF A-A2O3 SECURED THEREIN BY THE BINDINGACTION OF SAID IRON, AT THE SURFACE PORTION OF SAID ALLOY LAYER, SAIDSURFACE PORTION CONTAINING CHROMIUM WHICH ACTS AS A BINDER FOR SECURINGSAID SUBSTANTIAL LAYER OF A-AI2O3 IN COOPERATION WITH SAID IRON.
 2. Amethod as claimed in claim 1, wherein said alloy layer is 200 to 500microns in depth with said substantial layer of Alpha -Al2O3 extending10 to 80 mircrons in depth.
 3. A method as claimed in claim 1, whereinthe minimum heating time during the second heating step is more than 30minutes.
 4. A method for making ferrous metal having highly improvedresistance to corrosion and oxidization, which comprises coating aferrous workpiece with an aluminum alloy containing chromium in anamount within the range of 1 to 10 percent by weight based on the totalweight of said aluminum alloy, heating the coated ferrous workpiece forat least 30 minutes at an elevated temperature of from 700* to 930*C,and heating the thus treated ferrous workpiece at a predeterminedtemperature within the range of 950* to 1,350*C which is lower than themelting point of said ferrous workpiece, under an oxidizing atmospherefor not more than 10 hours, thereby forming an alloy layer uniformlyover the surface of said ferrous workpiece, said alloy layer containingiron and aluminum as its principle components and including asubstantial layer of a Alpha -Al2O3 secured therein by the bindingaction of said iron, at the surface portion of said alloy layer, saidsurface portion containing chromium which acts as a binder for securingsaid substantial layer of Alpha -Al2O3 in cooperation with said iron. 5.A ferrous workpiece prepared according to the method of claim 1, havinga surface portion formed with an alloy layer having relatively highresistances to corrosion and to oxidization, said alloy layer being 200to 500 microns in depth and containing iron and aluminum as itsprinciple components and including therein a substantial layer of Alpha-Al2O3 of 10 to 80 microns in depth secured in a surface portion of saidalloy layer, and wherein at least said surface portion of said alloylayer contains chromium which acts as a binder for securing saidsubstantial layer of Alpha -Al2O3 in cooperation with said iron.
 6. Aferrous workpiece as claimed in claim 5, wherein said alloy layerincludes a scattered Alpha -Al2O3 layer at a substantially intermediateportion of said alloy layer.